Differential amplifiers have long been known as basic modules for signal processing in virtually all fields of electrical engineering (for example Tietze, U., Schenk, Ch., “Halbleiterschal-tungstechnik”; Springer Verlag, 1986, 8th Ed., pages 66–72). They have two parallel paths, each having an amplifier transistor and a collector resistor. The lines connected to the emitters of the transistors are connected to one another. A current source for producing a quiescent current is connected into the common emitter line. Non-reactive emitter and base resistors are usually also provided in the circuit and serve the purpose of, for example, setting the operating point of the transistors. An input voltage applied between the base connections of the transistors is amplified to give an output voltage between the collector connections of the transistors.
Major features of differential amplifiers are the so-called small-signal gain and the large-signal response between the input voltage and the output voltage. The small-signal gain and large-signal response can be calculated using relatively simple mathematical expressions. The limit voltage in large-signal operation essentially depends on the quiescent current lb produced by the current source and on the resistance value RC of the collector resistor. The small-signal gain likewise depends on RC and lb as well as directly on the operating temperature T of the circuit and on the resistance value RE of the emitter resistor.
In principle, therefore, with differential amplifiers the small-signal gain is temperature-dependent and the limit voltage in large-signal operation is not, provided lb is not temperature-dependent. In many applications it is not acceptable for there to be temperature dependence. The values RE and RC are generally independent of the temperature. For temperature compensation in the small-signal response, the quiescent current lb has therefore until now been made deliberately temperature-dependent, as a result of which the influence of the temperature in the small-signal gain is compensated for and the small-signal gain is constant in a desired temperature range. The temperature dependence of lb, however, causes the limit voltage to be temperature-dependent in large-signal operation owing to the constant RC value. However, the two amplifier properties are often required to be stable independently of the temperature, for example in the case of dynamic compressors.